Reinforced plies are normally produced using extruders having an extrusion head comprising a die adapter inside which is housed a mandrel aligned with the output die and having an axial duct for supplying the reinforcing cords to the die. The front end of the mandrel facing the die usually presents an insert through which the cords are fed, and defining, together with the die adapter, an upper and lower channel by which the die is fed with plastic elastomeric material for forming two layers of elastomeric sheet material arranged on either side of an intermediate cord layer, and joined together along the caps between adjacent cords.
The insert usually consists of an upper and lower element gripped together in a contacting manner and defines a cord guide member for maintaining constant cord distribution as the cords are fed through the die and subjected to the lateral thrust of the plastic material supplied to the die along the said supply channels. On known extrusion heads of the aforementioned type, the upper and lower elements of the said insert present respective flat lateral surfaces gripped together in contacting manner and each having a series of grooves mating with corresponding grooves on the other surface so as to define respective cord guide holes.
Being located between the said two supply channels, the upper and lower elements of the said insert are therefore subjected to severe pressure, by which the said elements are compressed one against the other. Severe contact pressure is thus exerted on the contact surfaces of the said upper and lower elements, i.e., on the flat strip portions of the said contacting surfaces extending between each pair of adjacent grooves.
Obviously, therefore, for cord densities over and above a given value, the width of the said flat strips is so small s to produce specific contact pressures in excess of the yield point of the metal from which the insert is formed, thus collapsing the diaphragms between adjacent grooves on each element, and jamming the cord inside the guide member. On account of the above drawback, extrusion heads of the aforementioned type cannot be employed for cord densities involving strips of less than approximately 0.5 mm in width.